1. Field of the Invention
This invention relates to an injection mold for the manufacture of golf balls and to a method for preparing golf balls using the mold.
2. Prior Art
Most prior art methods for the manufacture of golf balls using molds employ injection molding and heat compression molding techniques. Especially in the case of golf balls consisting of a core and a cover, injection molding and heat compression molding techniques are typically used for molding the cover around the core.
The heat compression molding technique involves the steps of preforming hemispherical half cups from cover stock, enclosing a core with a pair of half cups, and placing the entirety in a heat compression mold where the ball is molded under suitable temperature and pressure conditions. In contrast, the injection molding technique produces a golf ball by placing a core in a cavity at the center and injecting molten stock material into the cavity through gates to mold the material around the core.
Referring to FIGS. 5A and 5B, there is illustrated an injection mold which is advantageously used when the cover stock is injection molded around a solid core or wound core. The illustrated example refers to the manufacture of two-piece solid golf balls. A typical prior art injection mold is illustrated as comprising a pair of separable upper and lower mold segments 1a and 1b. The mold segments 1a and 1b define a spherical cavity 2 when mated along a parting line PL. The mold includes a main runner 4, an annular cold runner 5 connected to the main runner 4, extending along the parting line PL and circumscribing the cavity 2, a plurality of circumferentially equally spaced nozzles 6 extending radially inwardly from the annular cold runner 5, and a corresponding plurality of gates 7 extending from the nozzles 6 and opening to the cavity 2. With this construction, a golf ball is manufactured by placing a core 3 at the center of the mold cavity 2, and supplying a cover stock based on a thermoplastic resin to the annular cold runner 5 through the main runner 4, thereby injecting the cover stock into the cavity 2 from the runner 5 through the nozzles 6 and gates 7.
From another aspect, it is desired for golf balls to have dimples evenly distributed on the spherical surface so that the ball may exert consistent performance regardless of shot positions. That is, golf balls which are improved not only in flight performance, but also in symmetry are desired. The demand for such improvements is increasing as the golf game becomes popular to a larger public. In order that high quality, high performance golf balls be delivered at a reasonable price, it is necessary to increase the cost performance, that is, the manufacturing efficiency of golf balls.
Golf balls, especially their covers are generally molded by injection molding and heat compression molding techniques as mentioned above. Most often, a mold comprising a pair of separable mold segments is used in either technique. A straight parting line is generally defined between the upper and lower mold segments. Corrugations are eliminated from the parting line for the purpose of simple manufacture of a mold. The straight parting line coincides with the equator line on the golf ball surface. As a consequence, since no dimples can be formed on the equator line corresponding to the parting line, the golf ball molded using this mold has a great circle band of a certain width along the equator line.
The golf ball having a wide great circle band along the equator line across which no dimples lie, however, has the problem that the ball exhibits varying flight performance depending on a particular impact position because the difficulty in uniformly arranging dimples on the spherical surface of the ball deteriorates the aerodynamic symmetry of the ball.
To overcome the problem, there were proposed several golf balls in which a wide great circle band on the equator line is eliminated and instead, dimples are formed on the equator line (see JP-A 173907/1986 and 143349/1994).
These proposals use a heat compression mold having no gates at the parting surface rather than an injection mold having gates at the parting surface. There have been proposed no injection molds which can effectively mold golf balls having dimples across the parting line.